1. Technical Field
This invention relates to vinyl halide copolymer compositions having good porosity properties. More specifically, this invention relates to copolymers of vinyl chloride and acrylated caprolactones which exhibit high porosity levels, high absorbtivity of plasticizer, easy processability and excellent moldability.
2. State of the Art
It is well-known that polyvinyl chloride (PVC) is one of the most useful materials known because of its superior chemical, physical and mechanical properties. It is also well established that PVC is one of the most modifiable plastic materials in existence. Consequently, PVC is used in large volumes because it can be modified to desired specifications to make specific molded end products. In choosing the method and materials for modifying PVC for particular molding applications, it is important that the chemical, physical and mechanical properties inherent in PVC be retained.
In one technique to improve upon the physical and mechanical properties of PVC, vinyl chloride monomer is copolymerized with various comonomers to achieve certain desired properties, such as improved processability. For example, it is well-known that the copolymerization of vinyl acetate with vinyl chloride results in a copolymer with improved processability over homopolymers of vinyl chloride. While this approach is advantageous in many applications, one of the problems encountered is that certain of the desirable properties inherent in the vinyl chloride homopolymer may be lost at the expense of the copolymerization. For example, in suspension polymerization, the PVC homopolymer particles produced are generally porous. Porosity is a very important property of the resin particles because it not only allows the polymerized resin particles to be easily stripped of its residual monomer thereby making it easier to comply with worker safety standards, but also gives the resin the ability to absorb large quantities of plasticizers. However, unlike PVC homopolymer, copolymers of vinyl chloride and vinyl acetate are "glassy" (e.g., lack porosity). Vinyl chloride copolymers made with 3 to 6 weight percent of vinyl stearate or other vinyl ester comonomers have been found to have very low porosity levels (below 0.10 ml/g as measured by mercury intrusion). In addition, unfavorable phenomenon such as the fusion of primary resin particles occurs during the polymerization reaction. Scanning electron microscopy (SEM) has revealed that a complete fusion of primary resin particles occurs during the copolymerization of vinyl chloride and vinyl acetate at conversion levels of 5 to 8 percent. Low resin porosities coupled with the fusion of primary resin particles leads to low level plasticized resins of irregular shape.
U.S. Pat. Nos. 3,945,958; 3,951,925 and 4,695,616 all disclose attempts to produce vinyl chloride polymers and copolymers having improved porosity. However, the methods disclosed therein require extremely specific suspending agents, catalysts and additives, as well as strict control over process conditions. No attempt is made to improve resin porosity through comonomer systems.
It would be highly desirable to provide a vinyl chloride copolymer resin with improved porosity and morphology properties that is easily polymerized using conventional polymerization additives and techniques. It would also be desirable to have a copolymer resin that is amenable to processing in the formation of molded articles.